For many commercial applications, semiconductor lasers capable of emitting radiation at relatively high power intensities, that is, more than several watts, are desired. Such lasers would be useful in applications such as light sources for long distance optical communications systems and optical recording systems. It might be naively thought that the power output from a semiconductor injection laser could be easily increased by increasing the current through the laser and thereby increasing the electron-hole recombination that leads to radiation. However, this solution is not feasible for more than a relatively limited current regime because the maximum power output obtainable from a single semiconductor laser is limited by the observed fact that catastrophic damage occurs to the cleaved mirror surfaces forming the optical cavity of the laser when the optical density exceeds a threshold value. The threshold for catastrophic damage varies from laser to laser but is typically between 3 and 5 MW/cm.sup.2. Accordingly, efforts have been directed toward devising semiconductor laser structures capable of emitting still higher power output.
One such effort has been termed the "large optical cavity laser" by those working in the art and is described in, for example, Applied Physics Letters, 17, pp. 499-502, Dec. 1, 1970. In the structure described, the recombination or active region, that is, the region in which electrons and holes recombine to yield radiation, is much smaller than the region in which the optical energy is confined, hence the name "large optical cavity." The large optical cavity permits higher radiation power to be emitted without catastrophic damage to the mirror surfaces as the radiation is emitted from the relatively large end surfaces of the large optical cavity. However, the large optical cavity lasers do not always utilize carriers with great efficiency as a significant number of carriers leave the active region and therefore do not contribute to the emitted radiation. Accordingly, effort was also directed toward optimizing, with respect to carrier utilization, placement of the active layer within the large optical cavity.
Another effort formed a device having a plurality of single heterostructure lasers comprising, for example, GaAs/Al.sub.x Ga.sub.1-x As, and solders them together. An improved version of this effort is described in Applied Physics Letters, 41, pp. 499-501, Sept. 15, 1982. In this embodiment, reversed biased p.sup.+ -n.sup.+ crystal tunnel junctions replace the solder joints.
However, the approaches taken by the latter two mentioned efforts suffer from several drawbacks. For example, the light-emitting, i.e., active, layers have to be relatively far apart to avoid material absorption by either the metallic joints or the tunnel junctions. This relatively large spacing means that the optical outputs from the individual lasers are not coupled to each other and are therefore incoherent and do not have the same spectral property. Additionally, if the device comprising the plurality of lasers is to have linear light output versus current characteristics, all of the lasers have to be essentially identical and must begin to lase simultaneously. This is an exceedingly stringent requirement that is often difficult to realize in practice.